The present research is focused on a fine-grained analysis of memory decline with aging, and the role of suppression mechanisms in age-related memory decline. Three groups of participants (continuous age ranges; young-old: 55-65 years, old: 66-75 years, and old-old: more than 75 years) were administered Forward and Backward span test, and a Working Memory task with Categorization (WMC). This new task requires lists of five words to be processed in order to individuate animal nouns, and that the last word of each list be contemporarily maintained. The words incorrectly recalled as target items, but presented during the task (intrusion errors), were computed in order to analyze the efficiency of suppression mechanisms. The findings indicated a continuous decline in working memory measures, and an early decline in short-term memory (passive storage) measures (between 60's and 70's). An age-related increase in intrusion errors was observed; the intrusion index was inversely related to working memory performance but, according to the hypotheses, was not related to short-term memory measures. These results suggest that the stronger working memory effect observed through age might be due to the combined influence of a decline in the capacity of short-term memory, and a loss of efficiency in suppression mechanisms.